The objective of this investigation is to establish the potential value for medical practice of antibody coated bacteria in counting B and T lymphocytes in blood smears. Escherichia coli was coated with anti kappa and anti lambda light chain antibody and the Ig bearing lymphocytes were labeled in washed whole blood cells. Smears were prepared and the cells having bacteria attached were counted. We found that lymphocytes of normal individuals bear either kappa or lambda chains on their surface indicating that they do not have passively absorbed surface Ig. We also investigated the possibility of using the spontaneous binding of some bacteria such as Brucella melitensis to identify B cells in blood smears. We found that B. melitensis bound to any B cell, normal or leukemic, and that it did not appear to bind to T cells. By either method 20% of the normal lymphocyte population (median value) were B cells. Attempts to prepare a potent and specific anti human T cell antiserum are made to establish by double labeling experiments that B. melitensis does not bind to any T cell. The principle of double labeling experiments with bacteria is based on the use of differently shaped organisms, e.g., a bacillus binds to B cells and a coccus to T cells. The variation of T/B cell ratio in the peripheral blood of normal individuals and of patients with hematological and/or immunological disturbances is investigated by these procedures. Using similarly prepared reagents we attempt to establish possible causes of changes in T/B cell ratios in experimental animals. BIBLIOGRAPHIC REFERENCES: Kleinman, Raphael, Dray, S. and Teodorescu, M. Separation of human lymphocyte subpopulations by their selective adherence of bacterial cell monolayers. Fed. Proc. 36: 1315, abstract No. 5542, 1977. E. P. Mayer, A. Bratescu, S. Dray and M. Teodorescu. Enumeration of human B lymphocytes in stained blood smears by their binding of bacteria. Clin. Immunol. and Immunopathol. 1977, in press.